Ferroelectrics that have a strong electro-optic effect are used for optical devices that convert an electric signal into an optical signal. For example, optical modulators that are configured by including a LiNbO3 (lithium niobate) substrate are widely in practical use. The optical modulator that is configured by including a LiNbO3 substrate is sometimes referred to as an LN modulator. Chirping is small in the LN optical modulator and the LN modulator can achieve high-speed modulation.
FIGS. 1A and 1B illustrate an example of the configuration of an optical modulator. FIG. 1A is a top view of the optical modulator seen from above. FIG. 1B depicts the optical modulator of FIG. 1A seen from the side.
A substrate 1 is a Z-cut LiNbO3 substrate that is formed in the Z-axis direction of a LiNbO3 crystal. An optical waveguide is formed in the vicinity of the surface of the substrate 1. For example, the optical waveguide is formed by introducing metallic impurities such as Ti in the vicinity of the surface of the substrate 1 and by diffusing the metallic impurities using heat. The optical waveguide includes an input optical waveguide 2a, a pair of straight optical waveguides 2b and 2c, and an output optical waveguide 2d. The straight optical waveguides 2b and 2c are optically coupled to the input optical waveguide 2a. In addition, the straight optical waveguides 2b and 2c are also optically coupled to the output optical waveguide 2d. That is, the optical waveguides 2a-2d form a Mach-Zehnder interferometer. Note that the straight optical waveguides 2b and 2c are formed substantially parallel to each other. In the following description, from among two surfaces of the substrate 1, a surface in which the optical waveguide is formed may be referred to as a “top surface” or a “mounting surface”. In addition, the other surface of the substrate 1 may be referred to as a “bottom surface”.
On the top surface of the substrate 1, a signal electrode 3 and a ground electrode 4 are formed. The material of the signal electrode 3 and the ground electrode 4 is, for example, gold. In the example illustrated in FIG. 1A, the signal electrode 3 is formed in the vicinity of one of the pair of straight optical waveguides 2b and 2c (in the example, the straight optical waveguide 2b). One end of the signal electrode 3 is electrically connected to an electric signal generator (not illustrated) and the other end of the signal terminal 3 is terminated using a resistor. Note that the electric signal generator generates an electric signal that represents transmission data. The ground electrode 4 is formed in an area where the signal electrode 3 is not formed, on the top surface of the substrate 1. In this example, the ground electrode 4 is formed reaching the area above the straight optical waveguide 2c. A buffer layer 5 is formed between the top surface of the substrate 1 and each electrode (the signal electrode 3 and the ground electrode 4). The buffer layer 5 suppresses light transmission from the optical waveguides (2a-2d) to the electrodes (3 and 4). Note that the buffer layer 5 is realized by an insulating film such as a SiO2 film.
The substrate 1 is cut out from a ferroelectric wafer by dicing. In this case, there is a risk of damaging the optical waveguide that is formed in the end of the substrate 1 (the input optical waveguide 2a and the output optical waveguide 2d depicted in FIG. 1A). Therefore, in order to protect an optical waveguide pattern that is formed in the substrate 1, a dummy block (protective member) is provided on the end of the substrate 1. In an example illustrated in FIGS. 1A and 1B, dummy blocks 6 are provided on an input end and an output end of the substrate 1. The input end means the end of the substrate 1 on the side in which the input optical waveguide 2a is formed. The output end means the end of the substrate 1 on the side in which the output optical waveguide 2d is formed. The dummy block 6 is formed of, for example, the same material as the substrate 1. The dummy block 6 also has the function of holding an optical fiber that is optically coupled to the optical waveguide of the optical modulator.
However, since the substrate 1 is a ferroelectric substrate, the substrate 1 causes a pyroelectric effect due to a temperature change. The pyroelectric effect leads to uneven distribution of electric charge. In a case in which the substrate 1 is a Z-cut substrate, electric charge is concentrated in the area in the vicinity of the top surface of the substrate 1 and the area in the vicinity of the bottom surface of the substrate 1. When the dummy block 6 is formed of a ferroelectric as with the substrate 1, electric charge may be concentrated in the dummy block.
Moreover, when a sharp temperature change occurs, uneven distribution of electric charge may become remarkable, resulting in electric discharging between the substrate 1 and the dummy block 6. The electric discharging between the substrate 1 and the dummy block 6 disturbs the electric field in the substrate 1 and thus decreases the quality of a modulated optical signal generated by the optical modulator.
Note that a technology for reducing uneven distribution of electric charge is proposed (for example, Japanese Laid-open Patent Publication No. 62-73207). An optical modulator that has the function of adjusting an operating point is known (for example, Japanese Laid-open Patent Publication No. 2003-233042). In a known configuration, a reinforcing block is provided at an end portion of a substrate at which an optical waveguide is formed (e.g., Japanese Laid-open Patent Publication No. 2009-258766). A technique has been proposed for suppressing uneven distribution of electric charge by increasing conductivity through a reduction treatment on the surface of a reinforcing block (e.g., Japanese Laid-open Patent Publication No. 2013-186200).
As described above, techniques have been proposed for mitigating a pyroelectric effect (i.e., uneven distribution of electric charge caused by a temperature change). In the prior art, however, the pyroelectric effect within the dummy block 6 is not mitigated. Performing, for example, a reduction treatment on the surface of the dummy block 6 increases the conductivity of the surface region, and this may possibly mitigates a pyroelectric effect. However, the reduction treatment on the surface alone does not mitigate the pyroelectric effect within the dummy block 6. Moreover, even if the surface of the dummy block 6 is reduced, the conductivity is decreased with time by oxidation.
Accordingly, in the prior art, when a sharp temperature change occurs, electric discharging may occur between the substrate 1 and the dummy block 6. That is, the problem of a decrease in quality of a modulated optical signal that is caused by a pyroelectric effect still remains.